It is known to identify in a fingerprint what is referred to as minutiae points, and identify or verify a person's identity using a plurality of such minutiae points' location, type and orientation in the fingerprint. In a first recording of a person's fingerprint a template is created, which constitutes reference data that is associated with the person's identity. This process is usually referred to as enrolment. The template can be stored electronically, either in a database of a plurality of people's identity, or on a data carrier, such as a smart card, carried by the person. A recorded fingerprint is frequently stored and processed as an image file, which is usually preprocessed, for example, by binarisation before it can be used. When a person's identity is to be verified or determined, an image of the person's fingerprint is recorded in a prior-art manner, usually by means of a silicon sensor. The image recorded is here referred to as “current fingerprint”.
In a “verification” of the person's identity, a current fingerprint is compared with a template, for the purpose of deciding whether the person having the current fingerprint is the person she pretends to be, i.e. the person with whom the smart card is associated.
In an “identification” of a person's identity, a current fingerprint is compared with a plurality of templates which are usually stored in a database.
In the following the expression “checking a person's identity” will be used to comprise both identification and verification.
Several techniques of representing fingerprints electronically are known. The above-described image file can be processed for identification of minutiae points. It is known to use minutiae points, the location and orientation of which in a person's fingerprint constitute unique features of the person. It is also known to identify minutiae points and to determine their type, location and orientation in a coordinate system. In a verification or identification, the minutiae points of a current fingerprint can be compared with minutiae points from one or more templates.
A variant of this method comprises selecting a reference minutiae point and representing the remaining minutiae points in relation to this reference point. This can make it easier to handle a situation where the orientation of the current fingerprint differs from the orientation of the template.
U.S. Pat. No. 4,135,147 discloses a method where a minutiae point is described in relation to minutiae neighbourhoods. In the method according to U.S. Pat. No. 4,135,147 a vector is created, which describes each minutiae point in terms of distance and angles in relation to the minutiae neighbourhoods. A template consisting of a plurality of such vectors can, in an identity check, be compared with a plurality of vectors which represent an unknown fingerprint. Since each minutiae point is described in relation to minutiae neighbourhoods, each such vector will contain a relative large amount of information which need be compared when checking a person's identity.
In a fingerprint 25-30 minutiae points are often found, sometimes up to 200. In the cases where a reference minutiae point is used, this is in most cases arbitrarily selected in the template. This may imply that all points in a current fingerprint must be tested as reference minutiae point, pairs of minutiae points being formed with each of all the other points, for the purpose of finding a match.
This method of representing fingerprints, however, requires a relatively large storage space, which is disadvantageous if it is desirable to store the fingerprint on a carrier with a limited storage space, such as a smart card.
Besides, the comparisons that must be made between a current fingerprint and a template are fairly complicated and take a long time. Thus, they are also less suitable for use in methods for checking a person's identity, in which a current fingerprint is to be compared with a large number of alternative previously recorded fingerprints.